The present invention relates to a rotary type electrostatic spray painting device.
One known electrostatic spray painting device which has been used for painting, for example, bodies of motor cars, is of the rotary type and comprises a rotary shaft supported by ball bearings or roller bearings arranged within the housing of the painting device, and a cup shaped spray head fixed onto the front end of the rotary shaft. In this painting device, a negative high voltage is applied to the spray head, and paint is fed onto the inner circumferential wall of the spray head. Thus, fine paint particles charged with electrons are sprayed from the spray head and are attracted by an electrostatic force onto the surface of the body of a motor car, which is grounded. As a result of this, the surface of the body of a motor car is painted. In such a rotary type electrostatic spray painting device, about 90 percent of the paint sprayed from the spray head can be efficiently used for painting the surface to be painted. Thus, the amount of the paint which is wasted is small and, as a result, rotary type electrostatic spray painting devices are used in various industries.
In order to form a beautifully finished surface when the surface is painted by using a spray paint, it is necessary to reduce the size of the particles of paint as much as possible. In rotary spray painting arrangements wherein the paint is divided into fine particles by operation of a centrifugal force which results from the rotation of the spray head, the strength of the centrifugal force, that is, the rotating speed of the spray head, has a great influence on the size of the particles of paint. In other words, the higher the rotating speed of the spray head, the smaller the size of the particles of paint. Consequently, in order to form a beautifully finished surface by using a rotary type electrostatic spray painting device, it is necessary to increase the rotating speed of the spray head as much as possible. As mentioned above, in a conventional rotary type electrostatic spray painting device, ball bearings or roller bearings are used for supporting the rotary shaft of the electrostatic spray painting device and, in addition, a lubricant, such as grease, is confined within the ball bearings or the roller bearings. However, when such bearings, which are lubricated by grease, are rotated at a high speed, the bearings instantaneously deteriorate. Therefore, in a conventional rotary type electrostatic spray painting device having bearings which are lubricated by grease, the maximum rotating speed of the rotary shaft, and therefore of the spray head, is at most 20,000 r.p.m. However, in the case wherein the rotating speed of the spray head is about 20,000 r.p.m., the size of the particles of paint is relatively large and, thus, it is difficult to form a beautifully finished surface. In the field of manufacturing motor cars, the painting process for bodies of motor cars comprises a primary spraying step, an undercoating step and a finish painting step. However, since it is difficult to form a beautifully finished surface by using a conventional rotary type electrostatic spray painting device, as mentioned above, such a conventional rotary type electrostatic spray painting device has been used for the undercoating step, but not the finish painting step.
A jet lubricating system is known wherein low viscosity lubricating oil is injected into the region between the inner race and the outer race of the ball or roller bearing. The friction between the ball or roller and such races is greatly reduced and, at the same time, the heat caused by the friction is absorbed by the lubricating oil. In arrangements where the above-mentioned jet lubricating system is applied to a rotary type electrostatic spray painting device, it is possible to increase the rotating speed of the rotary shaft of the electrostatic spray painting device as compared arrangements where grease lubricated bearings are used. However, since the jet lubricating system requires a complicated lubricating oil feed device having a large size, it is particularly difficult to apply such a jet lubricating system to a rotary type electrostatic spray painting device. In addition, if the lubricating oil mixes with the paint, the appearance of the painted surface is damaged. Therefore, if the jet lubricating system is applied to a rotary type electrostatic spray painting device, it is necessary to completely prevent the lubricating oil from leaking into the paint. However, it is practically impossible to completely prevent the lubricating oil from leaking into the paint and, thus, it is inadvisable to apply the jet lubricating system to a rotary type electrostatic spray painting device.
A further known painting device which is capable of reducing the size of the particles of paint to a great extent employs an air injection scheme in which the paint is divided into fine particles by the stream of injection air. In this air injection type electrostatic spray painting device, since the size of the particles of sprayed paint can be reduced to a great extent, as mentioned above, it is possible to form a beautifully finished surface. Consequently, the air injection type electrostatic spray painting device has performed the finish painting step for the bodies of motor cars. However, in such an air injection type electrostatic spray painting device, since the sprayed paint impinges upon the surface to be painted together with the stream of the injection air, and a large amount of the sprayed paint escapes with the stream of the injection air without adhering to the surface to be painted. In such systems, the amount of the paint used to effectively paint the surface to be painted is about 40 percent of the amount of the paint sprayed from the electrostatic spray painting device. Consequently, in situations where an air injection type electrostatic spray painting device is used, the consumption of the paint is inevitably increased. In addition, in this case, a problem occurs in that the paint escaping, together with the stream of the injection air, causes air pollution within factories.
It is, therefore, an object of the present invention to provide a rotary type electrostatic spray painting device capable of reducing the size of the particles of paint to be sprayed and the quantity of paint used.